Solid state light sources such as light emitting diodes (LEDs) generate visible or non-visible light in a specific region of the electromagnetic spectrum. An LED may output light, for example, in the blue, red, green or UV region(s) of the electromagnetic spectrum, depending on the material composition of the LED. When it is desired to construct an LED light source that produces light having a color different from that of the output color of the LED, it is achieved by down-converting the emitted radiation (“primary light”) from a LED source to radiations having longer wavelength (“secondary light” and “tertiary light” as needed) by a luminescent material via the process of photoluminescence.
Photoluminescence generally involves absorbing higher energy radiation (photons) and emission of radiation at longer wavelengths by a wavelength converting material (“conversion material”) such as a phosphor or mixture of phosphors. This process may be referred to as “wavelength conversion.” An LED combined with a wavelength converting structure that includes a conversion material such as phosphor to produce secondary or tertiary light may be described as a “phosphor-converted LED” or “wavelength converted LED” light source.
In a known configuration, an LED die such as a III-nitride die is positioned in a reflector cup package. To convert primary light from an LED to secondary light, partially or completely, a wavelength converting material must be included in a design of a light source. This material could be in the form of powder or a self supporting “plate,” such as a ceramic or single crystal plate. The plate may be attached directly to the LED die, e.g. by bonding, sintering, gluing, etc. Such configurations may be understood as “chip level conversion” or “CLC.” The “chip level conversion” also includes direct deposition of conversion material on the LED chip or deposition of a composite material consisting of particles of conversion materials dispersed in silicone. Alternatively, the plate may be positioned remotely from the LED. Such a configuration may be understood as “remote conversion.”
Additionally, it is always desired to utilize wavelength converting materials that can partially or completely convert primary light emitted by an LED to secondary and/or tertiary light to produce light with a desired correlated color temperature. In particular, interest has grown in the production of wavelength converting structures to fabricate “warm white” light sources, i.e., that the light generated by the light source is white and has a color temperature in the range of about 2500-3500K. Although warm white LED lamps have been developed using wavelength converting structures that include one or more phosphor powders dispersed in a polymeric (e.g., silicone) binder, such converters have many disadvantages. For example, powdered phosphor in silicone may impose limitations on the thermal management of an LED lamp, which may negatively impact lamp performance and lifetime.
Thus, there remains a need in the art for wavelength converting structures that can produce secondary and/or tertiary light with desirable optical characteristics, and which may avoid some or all of the limitations of a powdered phosphor in silicone converter.